System and method for sensing and mitigating hydrogen evolution within a flow battery system

ABSTRACT

A method is provided for mitigating hydrogen evolution within a flow battery system that includes a plurality of flow battery cells, a power converter and an electrochemical cell. The method includes providing hydrogen generated by the hydrogen evolution within the flow battery system to the electrochemical cell. A first electrical current generated by an electrochemical reaction between the hydrogen and a reactant is sensed, and the sensed current is used to control an exchange of electrical power between the flow battery cells and the power converter.

This application is a continuation of U.S. patent application Ser No.13/164,059 filed Jun. 20, 2011.

BACKGROUND

1. Technical Field

This disclosure relates generally to flow batteries and, in particular,to a system and method for sensing and mitigating hydrogen evolutionwithin a flow battery system.

2. Background Information

A typical flow battery system includes a flow battery stack, an anolytereservoir and a catholyte reservoir. An anolyte solution is circulatedbetween the anolyte reservoir and the flow battery stack. A catholytesolution is circulated between the catholyte reservoir and the flowbattery stack.

During operation, the flow battery stack may convert electrical energyinto chemical energy, and store the chemical energy in the anolyte andcatholyte solutions. Hydrogen evolution within the anolyte solution,however, may also occur as the electrical energy is being converted tochemical energy. The term “hydrogen evolution” describes a secondaryreaction where positively charged hydrogen ions combine with negativelycharged electrons to form hydrogen gas. The formation of hydrogen withinthe anolyte solution may decrease system efficiency and may also createan imbalance between the states of charge of the anolyte and catholytesolutions. It also may result in unsustainable changes to thecomposition of the solutions, which may require these solutions to bereplenished. There is a need for a system and method for sensing andmitigating hydrogen evolution within a flow battery system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a flow battery system;

FIG. 2 illustrates a cross-section of a flow battery cell;

FIG. 3 is a flow chart illustration of steps for charging a flow batterysystem in a manner that mitigates hydrogen evolution;

FIG. 4 is a graphical illustration of a first curve showing sensedelectrical current versus time, and a second curve showing electricalpotential of a first solution versus time; and

FIG. 5 illustrates an alternative embodiment flow battery system.

DETAILED DESCRIPTION

FIG. 1 illustrates a flow battery system 10. The flow battery system 10includes a first reservoir 12, a second reservoir 14, a first solutionflow circuit 16, a second solution flow circuit 18, a flow battery stack20, an electrochemical cell 22, a valve 24, a purge gas reservoir 26, apurge gas flow regulator 28, a power converter 30, and a controller 32.

The first reservoir 12 has an exterior reservoir wall 34 and contains afirst solution (e.g., a vanadium anolyte) within an interior reservoircavity 36, where the first solution has a first reversiblereduction-oxidation (“redox”) couple reactant (e.g., V²⁺ and/or V³⁺ions). The second reservoir 14 has an exterior reservoir wall 38 andcontains a second solution (e.g., a vanadium catholyte) within aninterior reservoir cavity 40, where the second solution has a secondreversible redox couple reactant (e.g., V⁴⁺ and/or V⁵⁺ ions).

The first and second solution flow circuits 16 and 18 each include asource conduit 42, 44, a return conduit 46, 48 and a solution flowregulator 50, 52, respectively. The solution flow regulator 50, 52 mayinclude a variable speed pump connected, for example, inline within thesource conduit 42, 44.

The flow battery stack 20 includes one or more flow battery cells 54.

FIG. 2 illustrates a cross-section of one of the flow battery cells 54shown in FIG. 1. Each flow battery cell 54 includes a first currentcollector 56, a second current collector 58, a liquid-porous firstelectrode layer 60, a liquid-porous second electrode layer 62, and aseparator 64 between the first and second electrode layers 60 and 62.The first electrode layer 60 may be an anode, and the second electrodelayer 62 may be a cathode. The separator 64 may be an ion-exchangemembrane (e.g., a Nafion® polymer membrane manufactured by DuPont ofWilmington, Del., United States). The electrode layers 60 and 62 arepositioned between the first and second current collectors 56 and 58.Additional examples of flow battery cells are disclosed inPCT/US09/68681, and U.S. patent application Ser. Nos. 13/084,156 and13/023,101, each of which is incorporated by reference in its entirety.

Referring to FIGS. 1 and 2, the source conduit 42 fluidly connects thefirst reservoir 12 to the flow battery stack 20 such that the firstcurrent collector 56 and/or the first electrode layer 60 in each flowbattery cell 54 receives the first solution. The return conduit 46reciprocally connects the flow battery stack 20 to the first reservoir12 such that the first reservoir 12 receives the first solution from thefirst current collector 56 and/or the first electrode layer 60 in eachflow battery cell 54. The source conduit 44 fluidly connects the secondreservoir 14 to the flow battery stack 20 such that the second currentcollector 58 and/or the second electrode layer 62 in each flow batterycell 54 receives the second solution. The return conduit 48 reciprocallyconnects the flow battery stack 20 to the second reservoir 14 such thatthe second reservoir 14 receives the second solution from the secondcurrent collector 58 and/or the second electrode layer 62 in each flowbattery cell 54.

Referring to FIG. 1, the electrochemical cell 22 may be configured as ahydrogen sensor. The electrochemical cell 22 includes a gas-porous firstelectrode layer 65, a gas-porous second electrode layer 66, a separator68, and a current sensor 70. The first electrode layer 65 may be ananode, and the second electrode layer 66 may be a cathode. The separator68 may be a proton-exchange or anion exchange electrolyte layer. Theseparator 68 is configured between the first and second electrode layers65 and 66 such that the electrode layers 65 and 66 and the separator 68may form a fuel cell 72. Other examples of a fuel cell are disclosed inU.S. Pat. Nos. 5,156,929 and 6,617,068, each of which is herebyincorporated by reference in its entirety. The current sensor 70 iselectrically connected between the first and second electrode layers 65and 66.

The valve 24 may be a one-way check valve. The valve 24 fluidly connectsthe first reservoir 12 to the electrochemical cell 22 and, inparticular, a top region of the interior reservoir cavity 36 to thefirst electrode layer 65. The first electrode layer 65 therefore islocated outside of the exterior reservoir wall 34 in the embodimentshown in FIG. 1.

The purge gas reservoir 26 contains purge gas such as, for example,nitrogen (N₂) gas. Other examples of purge gas include inert gases suchas carbon dioxide (CO₂) gas, Argon gas, etc.

The purge gas flow regulator 28 may include a variable speed pump or anelectronically actuated valve (e.g., a one-way valve). The purge gasflow regulator 28 fluidly connects the purge gas reservoir 26 to thefirst reservoir 12.

The power converter 30 may include a two-way power converter or a pairof one-way power converters. The power converter 30 may be configuredas, for example, a two-way power inverter or a two-way DC/DC converterconnected to a DC bus (not shown). The power converter 30 iselectrically connected to the flow battery stack 20. For example, thepower converter 30 may be electrically connected to the first and secondcurrent collectors 56 and 58.

The controller 32 may be implemented using hardware, software, or acombination thereof. The hardware may include, for example, one or moreprocessors, a memory, analog and/or digital circuitry, etc. Thecontroller 32 is in signal communication (e.g., hardwired or wirelesslyconnected) with the flow regulators 50 and 52, the current sensor 70,the purge gas flow regulator 28 and the power converter 30.

The flow battery system 10 may be operated in an energy storage mode tostore energy in the first and second solutions, or in an energydischarge mode to discharge energy from the first and second solutions.During both modes of operation, the controller 32 signals the solutionflow regulator 50 to circulate the first solution between the firstreservoir 12 and the flow battery stack 20 through the first solutionflow circuit 16. The controller 32 signals the solution flow regulator52 to circulate the second solution between the second reservoir 14 andthe flow battery stack 20 through the second solution flow circuit 18.The controller 32 also signals the power converter 30 to exchangeelectrical current with (e.g., provide electrical current to, or receiveelectrical current from) the flow battery stack 20 and, thus, the flowbattery cells 54 at a rate that corresponds to a selected currentdensity within the cells 54. The term “current density” describes aratio of (i) total current delivered to or drawn from the flow batterystack 20 to (ii) an active area (not shown) of one of the flow batterycells 54, and in particular, of the separator 64 (see FIG. 2).Alternatively, the electrical energy may be exchanged such that there isa substantially constant exchange of power between the power converter30 and the flow battery stack 20, or the voltage of the power convertermay be held constant. Or, any combination of galvanostatic,potentiostatic, or constant power modes may be utilized.

During the energy storage mode of operation, the electrical energyprovided to the flow battery stack 20 from the power converter 30 isconverted to chemical energy. The conversion process occurs throughelectrochemical reactions in the first solution and the second solution,and a transfer of non-redox couple reactants (e.g., H⁺ ions) from thefirst solution to the second solution across each of the flow batterycells 54 and, in particular, each of the separators 64. The chemicalenergy is then stored in the first and second solutions, which arerespectively stored in the first and second reservoirs 12 and 14. Duringthe energy discharge mode of operation, the chemical energy stored inthe first and second solutions is converted back to electrical currentthrough reverse electrochemical reactions in the first solution and thesecond solution, and the transfer of the non-redox couple reactants fromthe second solution to the first solution across each of the flowbattery cells 54. The electrical current is then provided to the powerconverter 30 from the flow battery stack 20.

Hydrogen evolution may occur within the first solution during the energystorage mode of operation when, for example, the first solution hasreached an especially high state of charge (e.g., greater thanapproximately 90% of the V⁺³ ions have been converted to V⁺² ions). Theterm “hydrogen evolution” describes a secondary reaction to the desiredenergy storage process where positively charged hydrogen ions combinewith negatively charged electrons. For example, instead of the desiredenergy storage reaction (2V⁺³+2e⁻→2V⁺²) occurring, the followingsecondary hydrogen evolution reaction occurs: 2H⁺+2e⁻→H₂. The electronsare produced by the reaction in the second solution (e.g.,2V⁺⁴→2V⁺⁵+2e⁻). Disadvantageously, the formation of hydrogen within thefirst solution may decrease system efficiency since the electricalenergy is not converted into the stored chemicals (i.e., the redoxcouples). Additionally, the secondary reaction can result in animbalance between the states of charge of the first solution and thesecond solution.

FIG. 3 illustrates a method for charging the flow battery system 10 in amanner that mitigates hydrogen evolution. For ease of explanation, thefollowing description begins with an assumption that (i) the state ofcharge of the first solution is less than approximately 80%, and/or (ii)little or no hydrogen evolution is occurring within the first solution.Referring to FIGS. 1 and 3, in step 300, the controller 32 signals thepower converter 30 to provide electrical power (e.g., constant current)to the flow battery stack 20 such that the flow battery cells 54 areoperated at a first energy input rate. The electrical power may becontrolled at a substantially constant current, power, and/or voltage.

In step 302, the controller 32 signals the purge gas flow regulator 28to provide purge gas to the first reservoir 12. The injected purge gascreates a positive pressure within the first reservoir 12 such that thepurge gas and other gases within the first reservoir 12 flow through thevalve 24 and into the first electrode 65. The positive pressure as wellas the valve 24 reduce/prevent a backflow of gas (e.g., air) fromentering the first reservoir 12 from the electrochemical cell 22. In analternative embodiment, step 302 may be omitted where overpressurecreated by the evolution of hydrogen pushes gases within the firstreservoir 12 through the valve 24 and into the first electrode 65.

In step 304, reactant (e.g., air) is provided to the second electrode66. The reactant may be provided via an electronically actuated reactantregulator (not shown), or by diffusion where the second electrode issimply exposed to ambient air.

Hydrogen may form within the first solution through hydrogen evolutionwhen, for example, the first solution has reached a relative high(e.g., >80-90%) state of charge. In step 306, the electrochemical cell22, which may be operated with a relatively low potential (e.g., 0.2volts) and/or a relatively low resistance across the electrodes 65 and66, generates an electrical current when hydrogen formed by hydrogenevolution is provided to the first electrode 65 along with the purgegas. The electrical current is generated through electrochemicalreactions on either side of the separator 68, as in a fuel cell 72.

In step 308, the current sensor 70 senses the electrical currentgenerated by the electrochemical reaction between the hydrogen and thereactant, and provides a current signal indicative of the sensedelectrical current to the controller 32.

In step 310, the controller 32 processes the current signal, andprovides a control signal to the power converter 30. The current signalmay be processed, for example, by comparing a value of the currentsignal to one or more threshold values. Each threshold value isindicative of a predetermined current signal value.

FIG. 4 is a graphical illustration of (i) a first curve 400 showing thesensed electrical current versus time, and (ii) a second curve 402showing electrical potential (vs. a hydrogen reference electrode) of thefirst solution versus time. An example of a threshold value is shown attime t₃₃ where the sensed electrical current becomes greater than zero,which corresponds to when hydrogen begins to form within the firstsolution. Another threshold value is shown at time t₃₆ (e.g., where thesensed electrical current is equal to approximately 1.8 amps), whichcorresponds to when the first solution becomes overcharged (i.e.,reaches approximately 100% state of charge).This is also where theelectrical potential of first solution levels off because the sidereaction (hydrogen evolution) begins consuming substantially all of thecurrent, which is not desirable and is why a method to detect thiscondition before it occurs is advantageous. FIG. 4 therefore illustrateshow the electrochemical cell 22 may detect hydrogen evolution before,for example, it becomes excessive.

The detection of hydrogen evolution within the first solution does notdepend on which flow battery cell, or cells, in the flow battery stackare generating hydrogen since the first solution (with hydrogen, ifgenerated) returns to the first reservoir 12. Whereas if one tries touse the flow-battery cell potential, one would need to measure the cellvoltages of each of the individual cells, as well as measure thehalf-cell potentials using a reference electrode. Such a method,however, requires a lot of instrumentation and data collection. Thecontrol signal is provided to the power converter 30 to control theexchange of electrical power between the power converter 30 and the flowbattery cells 54 as a function of the electrical current generated bythe electrochemical cell 22. For example, where the current signal valueis greater than or equal to one or more of the threshold values, thecontrol signal may be used to (i) iteratively or continuously reducepower (e.g., the current density) within the flow battery cells 54, or(ii) stop the exchange of electrical power between the power converter30 and the flow battery cells 54. The current density may be reduced,for example, to a predetermined level that corresponds to a respectiveone of the thresholds met by the current signal value. Alternatively,the current density may be reduced as a function of the electricalcurrent being generated by the electrochemical cell 22 such that as thecurrent signal value increases, the current density decreases.

The formation of hydrogen within the first solution due to hydrogenevolution may be mitigated using the aforesaid method. For example, whenthe current sensor 70 initially senses an electrical current caused byan electrochemical reaction between the hydrogen and the reactant, thecontroller 32 may signal the power converter 30 to stop providingelectrical current to the flow battery stack 20 to prevent additionalformation of hydrogen. In another example, when the current sensor 70initially senses the electrical current, the controller 32 may signalthe power converter 30 to reduce the electrical current being providedto the flow battery stack 20 to reduce the hydrogen evolution rate.However, once the current signal value is greater than or equal to aspecified threshold value (e.g., where the rate of hydrogen productionis considered to be excessive), the controller 32 may signal the powerconverter 30 to stop providing electrical current to the flow batterystack 20 to prevent the first solution from becoming overcharged andthus from generating excessive hydrogen. The aforesaid method may alsoimprove the safety of the flow battery system 10 by consuming thehydrogen gas produced by hydrogen evolution through the electrochemicalreaction within the electrochemical cell 22.

In some embodiments, the controller 32 may additionally or alternativelyprocess the sensed current signal to determine how much hydrogen isbeing formed within the first solution using Faraday's Law. The hydrogenthat is consumed in the first electrode 65, for example, is equal toI/2F, where I is the current sensed by the sensor 70 and F is Faradayconstant (96,485 coulombs/mol).

FIG. 5 illustrates an alternative embodiment of a flow battery system510. In contrast to the flow battery system 10 shown in FIG. 1, thefirst electrode 65 is located within the exterior reservoir wall 34,while the second electrode 66 remains located outside of the exteriorreservoir wall 34. This configuration may reduce the complexity of thesystem since, for example, the valve 24 may be omitted. The purge gasreservoir 26 may also be omitted since the likelihood of air enteringthe first electrode 65 and flowing into the first reservoir 12 isgreatly reduced as the first electrode 65 is sealed within the firstreservoir 12. The electrodes 65 and 66 and the separator 68 may also bedesigned and configured in such a way to ensure that most of the waterproduced drains back to the first reservoir 36, if desired.

While various embodiments of the flow battery system have beendisclosed, it will be apparent to those of ordinary skill in the artthat many more embodiments and implementations are possible within thescope of the flow battery system. Accordingly, the present flow batterysystem is not to be restricted except in light of the attached claimsand their equivalents.

What is claimed is:
 1. A flow battery system, comprising: a firstreservoir containing a first solution comprising a first reversibleredox couple reactant; a plurality of flow battery cells that receivethe first solution; and a hydrogen sensor connected to and in fluidcommunication with the first reservoir, wherein the hydrogen sensor isconfigured to receive gas from a top region of a cavity of the firstreservoir.
 2. The system of claim 1, wherein the hydrogen sensorcomprises an electrochemical cell that comprises a first electrode thatreceives hydrogen from the first reservoir; a second electrode thatreceives reactant; and a current sensor that senses electrical currentgenerated within the electrochemical cell by an electrochemical reactionbetween the hydrogen and the reactant, and provides a current signalindicative of the sensed electrical current.
 3. The system of claim 2,wherein the electrochemical cell further comprises one of a protonexchange electrolyte layer and an anion exchange electrolyte layer thatseparates the first electrode from the second electrode.
 4. The systemof claim 2, wherein: the first solution comprises an anolyte; the firstelectrode comprises an anode; and the second electrode comprises acathode.
 5. The system of claim 2, wherein the reactant comprises air.6. The system of claim 2, wherein the first electrode is located outsideof the first reservoir.
 7. The system of claim 6, wherein the firstelectrode is fluidly connected to the first reservoir through a one-wayvalve.
 8. The system of claim 6, wherein the first reservoir receivespurge gas such that the hydrogen within the first reservoir flows to thefirst electrode.
 9. The system of claim 2, wherein the first electrodeis located within an exterior reservoir wall of the first reservoir; andthe second electrode is located outside the exterior reservoir wall ofthe first reservoir.
 10. The system of claim 1, further comprising apower converter that exchanges electrical power with the flow batterycells as a function of a current signal provided by the hydrogen sensor.11. The system of claim 10, further comprising a controller thatreceives the current signal and provides a control signal to the powerconverter to control a rate at which the power converter exchangeselectrical power with the flow battery cells.